As a silicon single crystal manufacturing method, there has been conventionally known a method of pulling up a silicon single crystal ingot by a Czochralski method (which will be referred to as a CZ method thereinafter). According to this CZ method, a seed crystal is brought into contact with a silicon melt trapped in a quartz crucible, and the seed crystal is pulled upward while rotating the quartz crucible and the seed crystal, thereby forming an elongated neck portion at a lower portion of the seed crystal. Then, this portion is thickened to provide a fixed-diameter portion having a predetermined diameter by adjusting a rate of pulling and a temperature, and thereafter the crystal seed is pulled upward while rotating in accordance with crystal growth so that single crystal having a fixed diameter is grown. A crystal diameter of the single crystal which has reached a predetermined length is gradually decreased from the fixed-diameter portion, the diameter is eventually reduced to zero, and the single crystal is separated from the silicon melt.
Various ingenuities have been exercised with respect to such a CZ method in order to stably manufacture single crystal. For example, a crystal to be pulled upward is rotated around a central axis and also a quartz crucible in which a silicon melt is trapped is rotated in a direction opposite to that of the crystal, or a wire is used for pulling, or an SiO gas generated when a pressure of an inert gas is reduced is removed from an in-chamber atmosphere. Further, it is known that oxygen in a silicon single crystal produced by the CZ method plays a major part when manufacturing a device. For example, it is required in order to maintain strength of a silicon wafer. Furthermore, it is known that an oxygen precipitate generated by performing a heat treatment getters an impurity which is mixed from a surface of a silicon wafer. However, when a quantity of this oxygen is too large, an oxygen precipitate deteriorates device characteristics in the vicinity of a wafer surface. Therefore, an oxygen concentration must be controlled at a fixed level, but an oxygen concentration of an ingot pulled upward by the conventional CZ method is high on a top side of the ingot as shown in FIG. 12, and it is difficult to reduce the oxygen concentration when a quantity of a melt is increased.
In order to solve this problem, there has been known a method which pulls a silicon single crystal ingot upward from a silicon melt while applying a cusp magnetic field to the silicon melt. In order to generate this cusp magnetic field, an upper coil and a lower coil are arranged with a predetermined gap therebetween in a vertical direction outside a chamber in which a quart crucible is provided. Moreover, a cusp magnetic field which runs through a neutral plane between the upper coil and the lower coil from each coil center of the upper coil and the lower coil by passing currents opposite to each other to the upper coil and the lower coil. When the cusp magnetic field is generated, a reversed force (a Lorentz force) is applied to the silicon melt by an induced current produced in a direction vertical to the magnetic field, and free movement of the silicon melt trapped in the quartz crucible is thereby avoided. It is considered that a quantity of oxygen which enters the melt from the quartz crucible is reduced as a result of this phenomenon, whereby a quantity of oxygen which enters the crystal is lowered. However, an oxygen concentration is reduced with crystal growth like a case where a magnet is not used, and a rotating speed of the quartz crucible or a flow rate of an inert gas between a surface of the melt and a heat shielding member must be changed in order to uniform the oxygen concentration in a direction of a crystal axis.
On the other hand, in a process of manufacturing a semiconductor integrated circuit, as factors which reduce a yield ratio, there are a small defect of an oxygen precipitate which becomes a nucleus of an oxidation induced stacking fault (which will be referred to as an OISF hereinafter), a crystal originated particle (which will be referred to as a COP hereinafter) caused by a crystal, presence of interstitial-type large dislocation (which will be referred to as LD hereinafter) and others. As to OISF, a small defect which becomes a nucleus of a crystal is introduced during crystal growth, it is actualized in a thermal oxidation process or the like when manufacturing a semiconductor device, and it becomes a factor of a defect such as an increase in a leak current of the manufactured device. Additionally, the COP is a pit caused due to a crystal which appears on a wafer surface when a mirror-polished silicon wafer is cleansed by using a mixed liquid of ammonia and hydrogen peroxide. This COP becomes a factor which deteriorates electrical characteristics such as time dependent breakdown (TDDB) characteristics of an oxide film, time zero dielectric breakdown (TZDB) characteristics of an oxide film and others. Further, when the COP exists on a wafer surface, a step is generated in a wiring line process of a device, which can be a factor of disconnection. Additionally, it can be also a factor of leak or the like in an element isolation portion, thereby lowering a production yield. Furthermore, the LD becomes a factor which deteriorates electrical characteristics such as leak characteristics, isolation characteristics and others. As a result, it is necessary to reduce the OISF, the COP and the LD from a silicon wafer which is used in order to manufacture a semiconductor integrated circuit.
There has been proposed a silicon single crystal ingot manufacturing method which cuts out a defect-free silicon wafer which does not have the OISF, the COP and LD (see, e.g., Patent References 1 and 2). In general, a region [V] in which vacancy point defect agglomerates dominantly exist is formed in a silicon single crystal ingot when the ingot is pulled up at a high speed, and a region [I] in which interstitial silicon point defect agglomerates dominantly exist is formed in the ingot when the ingot is pulled up at a low speed. Therefore, according to the above-described manufacturing method, it is possible to produce a silicon single crystal formed of a perfect region [P] in which the above-mentioned point defect agglomerates do not exist by pulling up the ingot at an appropriate pulling rate.
Patent Reference 1: U.S. Pat. No. 6,045,610
Patent Reference 2: Japanese Unexamined Patent Application Publication No. 1393-1999.